Seismic detection

[Michael GilliganParticipant @michaelgilligan61133]

[Michael GilliganParticipant @michaelgilligan61133]

I’ve just read this fascinating piece about using in-situ fibre optic communications cables as seismic sensors **LINK**

https://scienmag.com/underwater-telecom-cables-make-superb-seismic-network/

It seems like a most ingenious ‘spin off’

MichaelG.

[John HaineParticipant @johnhaine32865]

I believe Crossrail tunnels and many modern bridges etc have optical fibres embedded in them to monitor small movements and vibrations.

**LINK**

[Michael GilliganParticipant @michaelgilligan61133]

Quite so, John … but those are custom installations

The innovation is to use the existing undersea telecomm fibres

MichaelG.

[John HaineParticipant @johnhaine32865]

But to be clear, what they have done is to use dark fibre that is not carrying telecoms traffic, looking at backscatter. Nice if the FOC you are using has spare fibre cores, but some obstacles to using an in-service fibre that may be carrying carrying two-way traffic already, and/or may have repeaters (depending on its length and what fibre was used and when it was made). The fibre used in the experiment was dedicated to serving an off-shore seismic station and was in its maintenance period. A very interesting technique in principle but a way to go I think before being widely deployed.

As a sidelight, I see from Wikipedia that fibre cables use a constant current drive along an associated conductor, a from a positive supply at one end and negative at the other. What they didn't say is that I believe there is another conductor laid at some distance from the fibre as a DC return path – spaced to avoid electrolytic effects. But as a result there is a very long loop that couples to the earth's magnetic field, generating a voltage as it changes. At times the voltages at either end may reverse and in effect the cable is powered by the earth.

[An OtherParticipant @another21905]

I have worked on a site which had optical fibres woven into its fencing, used as an intrusion alarm. It was simply connected across gateways by a buried fibre. My understanding was that the system was, in effect, calibrated by after installation such that it recorded the level of the received signal (input at one end via laser diode) – any flexing of the fibre caused the reflection and signal levels in the fibre to change, which triggered the detector. It worked fine.

[Clive HartlandParticipant @clivehartland94829]

When we took over a new building in Milton Keynes it had a vibration sensing circuit in the metal walls. I was called out time and time again with nothing found except the wall sensors tripping. Then i realised what it was, it was Silerstonne car race week and as the helicpters were bringing visitors they came over the building and the chopper blades set up a vibration enough to trigger the alarm. Short order to te alarm specialist who fitted it, cut it off!

Similar call out one night and they had smashed a window at ground level, entered and had taken a computer that was used for stock levels. what they missed was 5 Hewlet packard latest spec. computers that they used as a step coming through the window. Later when police came they contacted another police uint up the road who gad caught two chaps, queruing what was innthe boot of their car showed the exact items taken from our store.

[An OtherParticipant @another21905]

Found this today on the subject of Seismic detection:

**LINK**

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